Everybody here is forgetting something else...
While they do go out occassionally, like any other device,
automatics are much less maintenance prone.
Clutch changes, especially with an inexperienced manual driver, can get frequent.
As for shift abilities, just like Heidi said, you can shift automatics. I shift my automatics constantly. On the vette, I use it in place of the brake 3/4 times.
Some claim this accelerates wear, but I've never had to replace a Torqueflite, Turbo 350 or Turbo 400. (There is no true "manual" shift in an auto - even one with a full manual valve body. You are putting in a hydraulic "request" to the tranny's hydraulic "circuitry" to change the gear at the next appropriate time. No "missed" shifts or ground gears are possible since the hydraulic control system is deciding exactly when to do what.)
Automatics
may weigh more - I'm not sure. They also require a heavier cooling system with the tranny cooler which is much less efficient at removing engine heat because that cooler is in front of the radiator.
Before these Gear Vendors bolt on overdrives, there were no high power overdrive auto's - as John Z was saying. I'm right now considering a built up 700-R overdrive, but I know even the best will fail much sooner than the T400 3-speed in there. The 4L80 is a monster of a tranny - bigger dimensionally and weighing something like 130 pounds more than the 700R/4L60 and I think 100 more than the T400, so it wasn't super practical for a sportscar (although that certainly
would shift that weight rearward!
)
I admit I'm a sold auto guy, but I have always had the impression if you buy a high performance manual, be prepared to get that clutch adjusted every 3-4 months and changed every 18 if you drive it reasonably hard.
For the past week I've been driving mine without power steering, requiring two hands occassionally. THAT was enough to remind me why I like autos as well!